When a Democratic senator consistently gets 60% of the popular vote in a state with a moderate Republican heritage and Republican governors for the past 12 years, he should be considered a powerful asset. But if you’re one of the increasingly angry left-wing Democrats, you demonize him and then proceed with a primary.

That’s the position that a Connecticut senator and former vice presidential candidate, Joe Lieberman, finds himself in this year. Despite his more than 80% rating from Americans for Democratic Action and a low 20% rating from the American Conservative Union, he is being derided as a DINO – a Democrat In Name Only – by left-wing advocates and some party influencers, such as Howard Dean’s brother.

Amid President Bush’s increasing unpopularity in the state in which he was born (Mr. Bush was born in New Haven, Mr. Lieberman’s hometown), Mr. Lieberman’s support for the war on terrorism has made him persona non grata among left-wing Democrats. They have found their champion in a political neophyte and local businessman, Ned Lamont. One self identified Lamont delegate on the Daily Kos blog expressed the liberal rational for Mr. Lieberman’s replacement: “For years he has disparaged fellow Democrats and, time and again, provided a bi-partisan shield for Republicans’ harsh right-wing policies.”

And then, in the middle of Avlon’s column, comes this tantalizing tidbit:

Faced with this challenge, Mr. Lieberman’s recent statement that he would not rule out an independent candidacy if he loses the August primary set off waves of speculation. While it was an offhand comment, it underscores the fact that Mr. Lieberman could easily win as an independent. The two parties’ primaries are increasingly dominated by their more extremist voices, a situation unhealthy to the interests of a genuinely representative democracy.

Avlon goes on to talk about popular Republican moderates facing symmetrical ideological primary challenges from the right: Lincoln Chafee in Rhode Island, Congressman Joe Schwarz in Michigan. While the Club for Growth and pro-life groups are backing Chafee’s and Schwarz’s conservative opponents, Avlon notes that the same alliance failed to unseat moderate Arlen Specter in Pennsylvania. The moderate Republican Main Street Partnership has a fund-raising disadvantage relative to the passionate right, but is beginning to build a track record of backing winners like Specter. In a district with no Republican opponent, the conservative Club for Growth crossed over to support Texas Congressman Henry Cuellar, a fiscally conservative Democrat, in his successful primary fight against a left ideological candidate. Avlon concludes:

[True believers’] disproportionate influence in low-turnout primaries is yet another indication of the gap that exists between professional partisans and the general public. . . .

If centrist candidates like Mr. Lieberman are rejected by their party’s most activist elements and still run, the independent label may increasingly become a place of refuge for the sensible center, an opportunity to forge a new national consensus.

In other words, as both parties continue shrink and harden towards the extremes, centrist candidates and their many voters are increasingly being ejected or ejecting from the parties altogether. For the first time, it might be possible for independents in a wide range of races to begin winning elections.